The globin gene family has proved to be a very fruitful model system for the study of the structure, organization and control of the eukaryotic genome. Different hemoglobins, each composed of two Alpha-like and two Beta-like globin polypeptides appear sequentially in the circulation during the development of many animals. These changes are brought about by differential expression of the various globin genes, however, the mechanism by which individual genes become active at different times and to different extents is not well understood. We are using the human K562 cell line as a model system to study this problem. This cell line is capable of producing fetal and embryonic hemoglobins when treated with hemin in culture. To characterize and quantitate hemoglobin synthesis in K562 cells under various conditions, we are using gel electrophoresis and specrophotometric methods. The molecular basis of the effects of inducers of hemoglobin synthesis in K562 cells is being investigated at the level of globin mRNA production by RNA-DNA hybridization using specific cloned cDNA and genomic globin probes. Through these studies we are describing in detail some aspects of the process of gene activation in cultured cells.